This invention relates to a method for forming a plurality of valve metal capacitor bodies; that is, forming on the surfaces of the valve metal bodies, a thin film of valve-metal oxide that will serve as the capacitor dielectric in an electrolytic capacitor.
The valve metals most often used in electrolylic capacitors are tantalum and aluminum. Others such as titanium or the alloys of the above have not yet been of much commercial interest.
For making solid electrolyte capacitors, batch methods are used to start the process. For example, porous tantalum pellets each having a tantalum lead wire extending from one end are suspended by their lead wires in a liquid electrolyte. Typically, pellets are made by compacting tantalum powder in a mold with a tantalum wire partially buried therein and sintering to fuse the adjacent surfaces of wire and particles to each other. A cathode conductor is also submerged in the electrolyte. The tantalum oxide dielectric film is formed over the surfaces of the pellet by connecting an electric power supply between the cathode and the pellets, the latter serving as anodes in their electrolytic cell. Formed pellets of this kind are also placed into individual containers filled with a liquid electrolyte to make "wet" electrolyte capacitors.
On the other hand solid electrolyte capacitors are made by repeatedly impregnating the formed pellets with a manganous nitrate solution, heating to pyrolize the nitrate and convert it to manganese dioxide, a solid electrolyte. A counter electrode of graphite and silver may be formed over the manganese dioxide.
In any case, it will be appreciated that a most critical part of the process will be formation of the dielectric valve metal oxide film. It has become increasingly important to make such capacitors to tighter capacitance limits with high yields to minimize the waste of materials and to meet the growing demands by electrolytic capacitor users for close tolerance capacitors.
Forming pellets to a close capacitance tolerance is made difficult by the fact that the pelleting steps of compacting and batch sintering even under the best possible conditions produces pellets having a wide range of pellet surface areas.
From lot to lot, such pellets exhibit capacitance values over a wide range. Thus, it is customary to characterize each lot of pellets electrically prior to formation so that "large surface area pellets" may be predictably formed to a lower voltage than "small surface area pellets" to make both exhibit more nearly the same capacitance values.
The pre-formation testing is often called "wet-checking" and is accomplished by submersing a sample of the pellets from a lot into the electrolyte of a test cell and forming them at a fixed voltage V.sub.p for a fixed time. The capacitance is then measured at 1KH. From the well known fact that the product of the formation voltage V.sub.p and capacitance C.sub.p is about constant, this data leads to the particular C.sub.p V.sub.p product of this particular lot of pellets and the capacitance at any other formation voltage can be calculated to a first approximation. At wet checks the formation voltage V.sub.f that is associated with the desired capacitance C.sub.f is approximately determined by ##EQU1##
Even so, it is not possible to make the desired close tolerance capacitors directly from the wet check data alone. The above noted variations in pellets steming from pellet making variables as well as from the use of different batches of the start powder, leads to variations in the rate of formation from lot to lot of pellets. So, even when all the lots appear to have the same electrical characteristics prior to formation, there is still a substantial spread in their capacitance values after formation. Although very time consuming, this problem may be solved by periodically interrupting the formation, removing a sample number of pellets from the main formation tank and performing a wet check. Wet checks typically take from 5 to 20 minutes and are manpower intensive.
It is, therefore, an object of this invention to provide a faster more cost effective method for forming lots of valve metal capacitor bodies to tight capacitancevalue tolerances.